Vertebrate bone is a composite material composed of impure hydroxyapatite, collagen, and a variety of noncollagenous proteins, as well as embedded and adherent cells. Vertebrate bone can be processed into an implantable biomaterial, such as an allograft, for example, by removing the cells, leaving behind the extracellular matrix. The processed bone biomaterial can have a variety of properties, depending upon the specific processes and treatments applied to it, and may incorporate characteristics of other biomaterials with which it is combined. For example, bone-derived biomaterials may be processed into load-bearing mineralized grafts that support and integrate with the patient's bone or may alternatively be processed into soft, moldable or flowable demineralized bone biomaterials that have the ability to induce a cellular healing response.
The use of bone grafts and bone substitute materials in orthopedic medicine is well known. While bone wounds can regenerate without the formation of scar tissue, fractures and other orthopedic injuries take a long time to heal, during which the bone is unable to support physiologic loading. Metal pins, screws, and meshes are frequently required to replace the mechanical functions of injured bone. However, metal is significantly stiffer than bone. Use of metal implants may result in decreased bone density around the implant site due to stress shielding. Furthermore, metal implants are permanent and unable to participate in physiological remodeling.
Bone's cellular healing processes, using bone tissue formation by ostoblast cells coordinated with bone and graft resorption by osteoclast cells, permit bone grafts and certain bone substitute materials to remodel into endogenous bone that is almost indistinguishable from the original. However, the use of bone grafts is limited by the available shape and size of grafts and the desire to optimize both the mechanical strength and the resorption rate. Bone substitute materials and bone chips are quickly remodeled but cannot immediately provide mechanical support. In contrast, cortical bone grafts can support physiological stresses but remodel slowly.
U.S. Pat. Nos. 6,294,187, 6,332,779, 6,294,041, 6,123,731, 5,899,939, 6,478,825, 6,440,444, and 5,507,813, the contents of all of which are incorporated herein by reference, describes methods for preparing composites including allogenic bone for use in load bearing orthopedic applications. It is desirable to increase the strength of bone-reinforced composites by increasing the strength of the interactions with the matrix material.